Typically refers to the altering of a fixed-frequency carrier wave (radio or TV channel) in order to transmit data, audio or video from one location to another (see carrier
). At the receiving end, a tuner latches onto the particular carrier frequency and a demodulator circuit isolates the data from the carrier.
In traditional analog telephony, modulation alters a DC current. In such a conversation, both parties are modulating the current simultaneously.
Amplitude, Frequency and Phase
There are three basic types of modulation: amplitude modulation (AM) varies the voltage; frequency modulation (FM) varies the frequency, and phase modulation (PM) varies the angle of the wave (see amplitude modulation
, frequency modulation
and phase modulation
). In quadrature amplitude modulation (see QAM
), both the amplitude and phase of the carrier are modified.